Owing to its excellent mechanical strength, weatherability and chemical resistance, vinyl chloride polymer has been widely used in various industrial fields including the production of pipe and construction materials. However, its moldability is not very good and needs to be improved.
In general, the moldability of vinyl chloride polymer depends on the characteristics of the vinyl chloride polymer particles used. Therefore, in order to increase productivity, bulk density, and internal porosity, the diameter and particle distribution of vinyl chloride polymer particles needs to be controlled along with the improvement of gelation characteristics, plasticizer absorptiveness and powder fluidity.
In particular, it is necessary to increase the bulk density of vinyl chloride polymer to improve productivity from extrusion molding, and the gelation melting rate must be accelerated to improve the processability of extrusion molding. However, when extrudate produced from extrusion molding is increased, fusion torque is increased. But, fusion torque needs to be low for gelation of vinyl chloride polymer. It is also necessary to reduce processing time in order to increase polymerization productivity; in particular it is preferred to reduce polymerization time or to increase productivity per unit polymerizer.
The conventional suspension polymerization produces vinyl chloride polymer either with a high bulk density or with a fast gelation melting rate, which is not good enough for extrusion molding. That is, vinyl chloride polymer produced by suspension polymerization exhibits high polymerization productivity but low bulk density, or low polymerization productivity but high bulk density, suggesting that the effect of the polymer is in doubt because of its lowered polymerization productivity, smaller pores inside the polymer particles, or increased free polymer particles, even if vinyl chloride polymer with a high bulk density is produced.
As an alternative to improve the molding processability of vinyl chloride polymer, it has been proposed that 0.01˜0.2 weight part of one or more partially saponificated polyvinyl alcohols having 65 mol % hydration and an average degree of polymerization of 500˜4000 is added to 100 weight part of vinyl chloride monomer as a dispersion stabilizer for polymerization. This is followed by suspension polymerization, and then the reaction mixture is cooled down in a reflux condenser when the conversion rate of the vinyl chloride monomer reaches 3˜40%, during which a polyvinyl alcohol having at least 97 mol % hydration and an average degree of polymerization of at least 1500 is additionally added. However, the resultant vinyl chloride polymer of this proposed method has the problem of decreased melting properties because of the addition of a dispersion stabilizer having 90 mol % hydration, even though the bulk density of vinyl chloride polymer can be increased by this method.
There are two methods to improve the melting properties of a resin; one is the combination of two different initiators having different half-lives and the other is to improve the processability of a resin by regulating the polymerization degree distribution by changing the PVC polymerization temperature. Particularly, polymerization is performed in two steps; in step 1, polymerization is performed at a set temperature until the conversion rate reaches 50 weight %, and in step 2, polymerization of vinyl chloride monomer is performed at a temperature 5˜15° C. higher than the primary polymerization temperature. According to the above method, vinyl chloride having improved processability and high bulk density can be obtained, but productivity is reduced because of the longer time for the temperature change between the primary polymerization to the secondary polymerization. The polymerization temperature changes might result in side effects such as excessive coloring caused by sub-reactions between a reactant such as vinyl chloride monomer and an additive. Therefore, to compensate for such side effects by the sub-reactions, another additive such as a heat stabilizer has to be added which increases the production cost.
Therefore, a novel method of preparing vinyl chloride polymer with high bulk density is required, and at the same time having improved polymerization productivity.